The present invention is directed to a method and apparatus for plating lead frames used in fabricating integrated circuit devices and the like. In the fabrication of solid state devices, the lead frames are oftentimes heated when bonding a chip to the die-attach pad to form a gold-silicon eutechtic. When a lead frame is heated, the heat, depending on the metal used, oxidizes the metal. To avoid the effects of oxidation of the metal, and, in particular, to avoid the creation of an electrically insulating oxide on the metal surfaces, in the area of electrical contacts, it has been the practice to plate the electrical contact areas of the lead frames with gold or silver or like metal.
Initially, to have good electrical contact surfaces and form a gold-silicon eutectic, an entire lead frame or at least the entire surface of one side of the lead frame, in particular the side on which the semiconductor chip is bonded, was plated with gold. There was no selective masking of any parts of the frame. This type of plating used a great amount of gold.
Later, to conserve gold and thus reduce costs, as the cost of gold increased, the entire surface of the frame on the side on which the chip is attached was plated with silver and the use of gold was restricted in that it was selectively bonded to the die-attach pad alone in a two-step process.
In a still further development, the practice was to selectively restrict the plating of gold to the die-attach pad and to the lead tips so as to also conserve silver and gold.
In summary, in accordance with the prior described practices, lead frames have been provided with a gold-plated die-attach pad for forming the required gold-silicone eutectic and a silver or gold plated lead tip area free of oxidation created during fabrication for providing good electrical contacts.
With the still ever increasing cost of gold, however, the amount of gold used in plating each lead frame becomes an appreciable percentage of its cost and is, if unnecessary, a needless waste of an important natural resource.
It would seem a simple matter to plate the lead tips and die-attach area with silver and, thereafter, to plate the die-attach area with gold in a two-step process. However, it is found that when gold is plated over silver, the gold combines with the silver to the extent that as much as 66% more gold is required for forming a suitable gold-silicon eutectic than is required in the absence of silver as substrate plating.